Wideband bandpass filter based on ring resonator with high selectivity and multiple transmission zeros

A high-selectivity wideband bandpass filter consisting of a ring resonator and stepped impedance resonators is proposed. A wideband bandpass filter with multiple transmission poles and transmission zeros is obtained. For demonstration, a prototype operating at the frequency band of 4.14-9.56 GHz (with a fractional bandwidth of 79.1%) is designed and fabricated. Good agreements between the simulated and the measured results have been observed, indicating a very sharp out-of-band rejection.